Address family independence (2)

OS kernel support and applications support are usually separate
Need to ship a single binary that works on IPv6-enabled and IPv6-disabled kernel

IPv4 kernel + IPv4 userland
IPv4 kernel + IPv4/v6 dual stack userland
IPv4/v6 kernel + IPv4/v6 dual stack userland
"IPv6 code is present but not configured" case

-> "IPv6 support" is equal to "address family independent"
Application can handle both IPv4 and IPv6 without problem
Remove any code that assumes AF_INET